As a wild-type endonuclease of Serratia marcescens, a polypeptide comprising the amino acid sequence as shown in SEQ ID NO: 1 is known. This endonuclease is capable of degrading single-stranded or double-stranded DNA or RNA, without depending on its nucleotide sequence. Accordingly, such endonuclease is useful in a wide variety of biochemical fields. As with the case of E. coli, Serratia marcescens is classified as a Gram-negative bacillus. Accordingly, methods of producing a wild-type endonuclease derived from Serratia marcescens by culturing E. coli transformed with a vector comprising a nucleotide sequence encoding the wild-type endonuclease and allowing the endonuclease to be secreted into the culture supernatant have been known (Patent Literature 1; Non Patent Literatures 1 and 2).